Heat-shrinkable multilayer films comprising a core layer comprising an ethylene-vinyl alcohol copolymer, two outer layers comprising blends of ethylene-.alpha.-olefin copolymers and ethylene-vinyl acetate copolymers, and two adhesive layers, are known.
As an example U.S. Pat. No. 4,457,960 discloses the use of ethylene-vinyl alcohol copolymers and blends thereof in a multi-layer film which may be heat-shrinkable and comprise blends of ethylene-.alpha.-olefin copolymers and ethylene-vinyl acetate copolymers in the outer layers.
EP-A-87,080 relates to films having the above general structure and characterized by the fact that the ratio of the weight of the ethylene-vinyl acetate copolymer to the total weight of the polymers of the two outer layers is higher than 0.4.
EP-A-141,555 discloses an oriented five-layer film having a core layer of a blend of an ethylene-vinyl alcohol copolymer and nylon or nylon copolymer, two adhesive layers and two surface layers of a blend of ethylene-.alpha.-olefin copolymers and ethylene-vinyl acetate copolymers.
EP-A-217,596 describes a heat-shrinkable multilayer film comprising a cross-linked core layer comprising a blend of an ethylene-vinyl alcohol copolymer and a polyamide resin, two cross-linked inner layers each comprising an adhesive resin, and two cross-linked surface layers each comprising a three component blend of an ethylene-.alpha.-olefin copolymer of low density, an ethylene-.alpha.-olefin copolymer of medium density and an ethylene-vinyl acetate copolymer.
A general method of manufacture of the heat-shrinkable films, including the above ones, involves the co-extrusion of a thick tubular shape laminate film (called "tape") which is quenched just under the die, collapsed by a pair of nip rolls and then heated to a predetermined temperature above the glass transition temperature (Tg) but below the melting temperature of the composition and stretched in both the longitudinal (machine) and the transverse directions to get a cylindrically-shaped laminate thin film. After being so stretched the film is rapidly cooled to somehow freeze-in the resulting film a latent shrinkability. The film thus obtained, when heated to a temperature close to the temperature at which it has been stretched (or oriented) under conditions where nil or negligible restraint to inhibit shrinkage is present, will exhibit a rapid and irreversible reduction in its linear dimensions.
All the heat-shrinkable multilayer films indicated above are claimed to have very good properties in terms of oil-resistance, peel-resistance, gas-barrier, optics and shrink properties, the combination of which is highly desirable in the field of food packaging.
Most of these remarkable properties are peculiar to the specific compositions, the sequence of the layers and the thicknesses thereof. Some of them depend on a specific layer while others on the specific combination of layers. As an example, oil resistance is mainly ascribable to the presence of an ethylene-.alpha.-olefin component, in the outer layers, while the gas-barrier properties depend on the presence of an ethylene-vinyl alcohol copolymer in the core layer and, owing to the moisture susceptibility of this copolymer, also on the presence of moisture impermeable outer layers.
As far as the shrink properties are concerned, these will however depend on both the composition of the films and the process of manufacture involved in their production. As a matter of fact the stretching or orientation in the different directions may be imposed on the films in varying degrees and the shrink which will be recovered will clearly depend on that.
In general, it is however possible with the above compositions, by properly adjusting the manufacturing process parameters, to get films with a very high free shrink compared to other compositions. A particularly high free shrink is one of the key attributes of a film for food packaging as it provides the package with a skin-tight wrapping, which remarkably improves the appearance of the package and enhances the commercial value of its content.
To exploit all the above properties, the above films should have a thickness of at least 15 .mu., preferably at least 18 .mu., wherein the thickness of the gas-barrier layer is at least 3 .mu., preferably at least 4 .mu., and an orientation ratio, i.e., the multiplication product of the extent to which the tape is expanded in two directions perpendicular to one another, of at least 2.times.2, and preferably at least 2.5.times.2.5 to provide the desired high free shrink.
However coupled with such a high free shrink these films also show a very high shrink force, i.e. during the shrinking process they release a very high force.
In some cases, and chiefly when these films are employed for packaging soft articles, a high shrink force, particularly in one direction, may create severe problems as it may lead to a distortion of the packaged articles.
As an example, a typical application of the heat-shrinkable multilayer films having a core layer comprising an ethylene-vinyl alcohol copolymer, two outer layers of blends comprising ethylene-vinyl acetate and ethylene-.alpha.-olefin copolymers, and two adhesive layers, such as for instance those films marketed by Cryovac.RTM. under the tradename BDF, is in the modified atmosphere packaging (MAP) of food products placed on trays or otherwise flexible lower support members.
Briefly, in this packaging system, the food product in the tray is wrapped into a film envelope under a gas flushing. The excess gas is removed from the envelope by means of a slight pressure on top of the package (to avoid the ballooning effect during shrinkage), the open end of the envelope is then sealed, and the loose package is passed into a hot air shrink tunnel set at a temperature of from 140 to 160.degree. C., for a short time sufficient to let the film reach a temperature of about 120.+-.10.degree. C., to provide a tight package. Under these conditions the known films will shrink by at least 40% in both dimensions but at the same time they will release a very high shrink force. Depending on the strength of the tray employed, the high shrink tension in the transverse direction may lead to a more or less severe distortion of the tray that in any case impairs the package appearance.
Another typical application for this type of barrier films is in the MAP packaging of food products, such as pizzas, wherein the pizza base acts as the package support. The same packaging process is used with the only difference that there is no tray.
It has been found that for these types of application, in view of the trays currently available on the market, a suitable shrink force in the transverse direction should not exceed 0.5 N/cm, in order to control distortion of the tray or of the product.
One possible solution to this problem would involve a modification of the parameters of the orientation step in the process for the manufacture of the films. In this way the reduction in transverse shrink force would bring about also an undesirable and remarkable reduction in the free shrink of the end films.
An alternative solution might provide for the reduction of the film thickness below 15 .mu., with an obvious consequent reduction of the film barrier and mechanical properties.
As a third alternative solution a modification of the composition of these films might be envisaged with the aim at getting films with a reduced shrink force, but this would clearly bring about a modification also of the other properties of the films which on the contrary have been optimized in the above structures.